Life at the periphery of the Local Group: the kinematics of the Tucana dwarf galaxy

Aims: Dwarf spheroidal galaxies in the Local Group are usually located close to the Milky Way or M 31. Currently, there are two clear exceptions to this rule, and the Tucana dwarf galaxy is the most distant at almost 1 Mpc from the Milky Way. Our aim is to learn more about the nature of Tucana by measuring its radial velocity and internal kinematics. Methods: Using the VLT/FORS2 spectrograph in multi-object mode we were able to measure the velocities of 23 individual red giant branch stars in and around Tucana using the Ca II triplet absorption lines. From this sample 17 reliable members have been identified. Results: We measured the systemic velocity and dispersion of Tucana to be v_hel = +194.0±4.3~ km s-1 and σ_l.o.s.=15.8+4.1-3.1~ km s-1, respectively. These measures are obtained after removing the signature of rotation using a linear gradient of 6.5 × R/R_core± 2.9~ km s-1 which corresponds to a rotation of ≈16 km s-1 at the reliable limit of our data. Our systemic velocity corresponds to a receding velocity from the barycentre of the Local Group of v_LG = +73.3~ km s-1. We also determined the mean metallicity of Tucana to be [Fe/H] ~= -1.95±0.15 with a dispersion of 0.32±0.06 dex. Conclusions: Our study firmly excludes any obvious association of Tucana with the H I emission in the vicinity and shows that Tucana is a genuine dwarf spheroidal with low metallicity stars, no gaseous ISM and no recent star formation. The present location and relatively high recession velocity are consistent with Tucana having been an isolated Local Group galaxy for the majority of its existence.